The present invention relates to a composite material and to a method of manufacture thereof, and more particularly relates to a composite material, made up of a mass of silicon carbide reinforcing whiskers embedded within a matrix of metal, which has improved physical characteristics including wear resistance and tensile strength, and to a method of manufacture thereof.
In motor vehicles and aircraft and so forth, nowadays, the constant demand for lightening and strengthening of structural members and parts, with the objects, among others, of providing energy savings by reducing fuel consumption, and also of providing higher traveling speeds, has meant that construction from light alloy such as aluminum alloy or magnesium alloy has become common. Problems arise, however, in making parts from aluminum or magnesium alloys, despite the light weight of these aluminum or magnesium alloys, and despite their easy workability, because the mechanical characteristics of these alloys such as wear resistance, and such as strength including bending resistance, torsion resistance, tensile strength, and so on are inferior to those of competing materials such as steel. Further, the occurrence of cracking and the spreading of cracks in parts made of aluminum or magnesium alloy can be troublesome. Therefore, for parts the strength of which is critical there are limits to the application of aluminum and magnesium alloys.
Accordingly, for such critical members, it has become known and practiced for them to be formed out of so called two phase or composite materials, in which reinforcing material is dispersed within a matrix of metal. If the matrix metal is an aluminum or magnesium alloy, then the advantages with regard to weight and workability of using this aluminum or magnesium alloy as a constructional material can be obtained to a large degree, while avoiding many of the disadvantages with regard to low strength and crackability; in fact, the structural strength of the composite materials made in this way can be very good, and the presence of the reinforcing material can stop the propagation of cracks through the aluminum or magnesium alloy matrix metal.
The reinforcing material conventionally has been known as for exmple being alumina fibers, carbon fibers, silicon carbide whiskers, or possibly mixtures thereof, and the matrix metal has been known as for example being various types of aluminum or magnesium alloy; and various proposals have been made with regard to compositions for such fiber reinforced metal type composite materials, and with regard to methods of manufacture thereof.
Now, however, since such reinforcing fiber materials are vastly harder than the aluminum or magnesium alloy matrix metal in which they are embedded, therefore machining operations and finishing operations for parts formed of such composite materials including such reinforcing fiber materials become much more difficult. Further, during use of the parts, other problems occur: for example, the wear on mating or cooperating members which rub against such parts, sliding relatively to them, may become very great.
Ironically, although the use of silicon carbide whiskers as reinforcing material has appeared to be very promising, since this material is compatible with aluminum alloys which can be thus conveniently used as matrix metal, and since such silicon carbide whiskers have very good rigidity and strength and thus would be very suitable as reinforcing material, these problems associated with wear on a mating or cooperating member are particularly marked in such composite materials including silicon carbide whiskers. Specifically, in a mass of silicon carbide whiskers, because of the method of manufacture thereof, there is generally contained a certain considerable amount of non whisker or fiber shaped silicon carbide particles, which are usually spherical or irregular in shape, of various sizes; the percentage by weight of these non whisker particles, i.e. shot particles, may typically be from between 5% to 50% by weight. The diameters of these non whisker type silicon carbide particles are generally much larger than the diameters of the silicon carbide whiskers, such as several tens to several hundreds of times said whisker diameters, and, since their hardness is second only to that of diamond, being a hardness Hv (50 gms) of at least 1000, it will be readily understood that the working, machining, and finishing of the composite material including such silicon carbide particles become extremely difficult, and that also problems arise with respect to the wear on mating or cooperating members which rub against parts made of such composite material. Further, because it can often occur that such non whisker shaped particles become dislodged from the matrix metal in which they are embedded, scuffing of the material of such mating or cooperating members may well occur, which can cause great damage to such members.
With regard to methods of manufacture of composite materials, various methods of manufacture have been tried, but of these the most generally and usefully applicable has so far been the high pressure casting method, in view of the low cost of the fiber reinforced metal type composite material produced thereby, and the manufacturing efficiency attained thereby. In this high pressure casting method, a mass of reinforcing fibers is placed in the mold cavity of a casting mold, and then a quantity of molten matrix metal is poured into the mold cavity. The free surface of the molten matrix metal is then pressurized to a high pressure such as approximately 1000 kg/cm.sup.2 by a plunger or the like, which may be slidingly fitted into the mold. Thereby the molten matrix metal is intimately infiltrated into the interstices of the mass of reinforcing fibers, under the influence of this pressure. This pressurized state is maintained until the matrix metal has completely solidified. Then finally, after the matrix metal has solidfied and cooled into a block, this block is removed from the casting mold, and the surplus matrix metal around the reinforcing fibers may be removed by machining, so that the composite material mass itself, consisting of the mass of reinforcing fibers impregnated with matrix metal, is isolated. This high pressure casting method has the advantage of low cost, and it is possible thereby to manufacture elements of different shapes including quite complicated shapes with high efficiency.